The present invention relates to the growth of plants in unconventional media.
In order to grow, plants require adequate water, light, appropriate temperatures, oxygen, and gas exchange with their root tissue. In natural or near-natural environments, these needs are met by sunlight, rainfall, the local climate, and soil. In certain well-understood artificial environments, such as a greenhouse or other indoor growth facility, sunlight may be replaced or complemented by artificial light, rainfall can be replaced with manual or automatic irrigation, the temperature can be controlled artificially, and the soil (or a soil substitute) and fertilizers can be selected and blended for optimum growth.
Other environments present more difficult challenges. In “outer space” (i.e., earth-orbital or beyond-orbital locations), plants, animals and human beings can only survive in artificial environments that provide oxygen, water, and nutrition. Since the beginning of the space age, scientists have obviously recognized that long term human presence in orbital or beyond-orbital locations will require a corresponding long-term food supply sufficient to maintain the good health of space travelers (astronauts).
One solution is, of course, to carry food supplies from the Earth's surface to the orbital or beyond-orbital destination. For orbital missions the necessary food is typically either included with the original launch or replenished with additional travel to orbital locations; e.g. shuttle flights to the ISS. Even lunar missions (e.g., Apollo) can operate to some extent using this system. Although this has been the method of choice since the beginning of manned spaceflight in the early 1960s, it is phenomenally expensive; e.g., a cost of approximately $10,000 per pound at current prices to reach Earth orbit. Destinations beyond Earth orbit are obviously increasingly more expensive.
For example, the current International Space Station (ISS) typically sustains three crew members who require approximately 4 tons of supplies every six months. This means that launching such supplies costs approximately $80 million at current prices, not including the cost of the supplies or their preparation.
Manned missions beyond earth orbit, such as missions to Mars, will, however, require greater—much greater—amounts of life-sustaining supplies, including food. By way of comparison, using current technology, travel from the Earth to the Moon takes approximately 3 days, while travel to the nearest planetary neighbor (Mars) is expected to take at least six months and potentially longer.
Accordingly, significant interest exists in techniques for cultivating plants in space and doing so in a manner that can provide a partial or completely self-sustaining food supply over extended periods of time. In one sense, a plant represents a natural nanotechnology in which a very small, minimal-weight item—a seed—has the potential to produce enormously larger amounts of food and to also produce more seeds from which more food can be grown.
Additionally, plants are part of the natural cycle that converts carbon dioxide into oxygen. As a result, the growth of plants in a space environment has the potential to at least complement and potentially replace artificial technology for removing carbon dioxide from the atmosphere and to similarly complement or replace the need to carry oxygen.
Conducting agriculture in space on a large scale, however, raises problems with respect to both the space technology and the plants themselves. A spacecraft represents a sealed environment, operates in a microgravity or zero-gravity environment, and depends upon sophisticated mechanical and electronic components. In many circumstances, water and typical agricultural chemicals (fertilizers, acids, nitrates, phosphates) are likely to react unfavorably with a number of such electronic and mechanical items, thereby reducing (or destroying) their performance capabilities.
Apart from the previous problems, a low or zero gravity environment also raises issues with respect to the normal cultivation of plants. For example, water diffusion is quite different in a low or zero gravity environment than on Earth. On earth, gravity is the dominant force acting on water diffusion. Under zero or reduced gravity, however, capillary forces become dominant and thus create water distribution patterns different from those on Earth.
The lack of gravity is also likely to alter the nature of liquid and gas exchange between plant roots and their growth medium (e.g., soil in a natural environment; a supplemented soil or soil substitute in a space environment). Gas exchange within a growth medium is, however, typically a factor in plant root growth. Normally, soil, or another porous medium provides support for the root network and facilitates the supply and storage of liquids and nutrients to the plant. Roots also respirate to a certain extent and thus contribute to the exchange of carbon dioxide and oxygen. Plants also physically react to both gravity and light.
Additionally, constraints on the physical space available in a spacecraft (or any other confined space) tends to encourage the use of agriculture in containerized systems where plant roots are restricted to relatively small volumes, particularly in comparison to the space that would be available in native soil. Such restricted volume reduces the water storage capacity of the growth medium, reduces the surface area for root absorption, and tends to create a water table at the bottom of the container that can raise aeration problems.
Low gravity also changes the nature of gas exchange. On Earth, when plants are grown in porous substrates (soil or soil replacement) the media tends to drain easily after watering to in turn provide air filled space that permits gas exchange with the root tissues of the plant. In the absence of gravity, some other technique must be used to drain the growth media in a manner that complements the needs of the root and the remainder of the plant.